Variable-pitch nozzle for a radial turbine, in particular for an auxiliary power source turbine

ABSTRACT

A radial turbine nozzle of a turbine engine rotating about a central axis includes a first annular array of fixed blades and a second annular array with a same number of variable-pitch blades. The blades have pressure and suction surfaces. Each variable-pitch blade is connected to cups and is configured to be rotated by a controller about a geometric axis connecting centers of the cups. Each variable-pitch blade is mounted at a distance from the axis of the cups such that this axis of rotation is positioned facing the suction surface of the blade and substantially closer to a trailing edge than to a leading edge of that blade. The nozzle can modify the reduced flow admitted by a radial turbine in accordance with requirements of a thermodynamic cycle and produce a seal in an area of maximum load of the nozzle blades.

TECHNICAL FIELD

The invention relates to a variable-pitch radial turbine nozzle andparticularly, but not exclusively, a nozzle for a turboshaft engine orauxiliary power source turbine.

The field of the invention is that of gas distribution in the turbinesof turbine engines and, particularly, adapting fluid flow to reduce fuelconsumption, particularly specific fuel consumption (abbreviated to Cs)under partial load, and improving the operability of engines,particularly turboshaft engines or auxiliary power units (abbreviated toAPUs). The term turbine engine refers to turboshaft engines, APU-typeunits and turbochargers.

An APU is an energy source that makes it possible, for instance, tostart the main engines of aircraft and provide non-propulsive power(cabin pressurisation power, electrical and/or hydraulic power). Somesecure APUs can also act while in flight, in the case of engine failure,to attempt to restart the engine and/or to provide power to equipment.

A turboshaft engine or an APU is generally composed, first of a singleor dual primary shaft, on which are mounted, on the one hand, compressorstages (high- and low-pressure, hereinafter HP and LP, for a two-spoolor just HP for a single-spool engine) and, on the other hand, turbines(HP and LP or solely HP), and a secondary shaft on which an LP powerturbine is mounted. The power turbine is formed of rotor blade discs anddiscs with stator blading or nozzle. The turbines can be radial, withinward flow of gases. In this case, the stator blading is mounted on theperiphery of the rotor blading. The nozzle makes it possible to regulatethe gas flow by deflection, using stator blades.

The compressor and the HP turbine, linked to a combustion chamber, formthe gas generator. In operation, the compressed air is mixed with thefuel in the chamber, leading to combustion. The exhaust gases are thenpartially expanded in the HP turbine (or the HP and LP turbines) so asto drive the compressors, then in the power turbine via the nozzle.

The power turbine is coupled to direct drive means for equipment (loadcompressor, fuel and hydraulic pumps, electrical generator and/orelectrical starter/generator etc.), or via a power transfer box withadaptation of rotation speeds. Air taken at the outlet of the loadcompressor or turboshaft engine compressor can be used for cabin airconditioning and/or for main engine air start.

BACKGROUND ART

A fixed-geometry turbine engine has the disadvantage of havingunattractive thermal efficiency under partial load. Indeed, the engineis conventionally designed for optimal operation in conditions close toits mechanical and thermal limits. When it supplies power very muchbelow these optimal points, the compression rate and the temperature arethen substantially lower, as is the compression efficiency, in general.This leads to thermal efficiency very much inferior to that of thetheoretical value, and therefore to mediocre specific consumption—i.e.,fuel consumption per unit of power.

One possible solution to mitigate this effect is to use variablegeometry. In this case, in order to lessen the airflow passing throughthe engine without excessively reducing the compression rate or thecombustion temperature, the delivery section of the high-pressureturbine—located just downstream of the combustion chamber—is decreasedby using variable-pitch blades for the stator (called the nozzle in aturbine).

It is also possible, on a civilian aircraft, to envisage exploiting thepressure energy available in the pressurised cabin by installing aturbine in the air discharge opening (cabin air being constantly renewedfor passenger safety, at a pressure exceeding external ambientpressure). The outlet port is generally a variable-section valve, slavedto the cabin pressure control system.

Such a turbine must, just like a conventional valve, be able to ensure avariable reduced rate in accordance with pressure settings produced bythe cabin pressure control system, and on the pressure differencebetween the cabin and the exterior (defining the expansion ratio of theturbine). Here too, a variable-section turbine nozzle managed byvariable-pitch nozzle blades is one solution.

DISCLOSURE OF THE INVENTION

The object of the invention is to improve the mechanical strength of thenozzles and the overall efficiency of the turboshaft engine. To thisend, it proposes to produce a nozzle incorporating variable-pitch bladesto regulate and control the gas flow rate, it being possible to rotateeach blade into a specific position. To improve the performance of thenozzle, the seal between the nozzle blades and their spacing system isproduced in the area of maximum load of the nozzle blades. This sealthen makes it possible to limit any unwanted gap flows in the area wherethey would be most intense.

More specifically, the invention relates to a radial turbine nozzle fora turbine engine, rotating about a central axis, and comprising a firstannular array of fixed blades and a second annular array with the samenumber of variable-pitch blades, the blades having pressure and suctionsurfaces. Each blade in the second array, which is rigidly connected tocups extending at each end of the blade facing the pressure and suctionsurfaces of the blade, is capable of being rotated by pitch controlmeans about a central geometric axis connecting the centres of the cups.Each of these blades has a trailing edge and a leading edge for theflows of gas linked to the pressure and suction surfaces, it beingpossible and advantageous for the leading edge of each variable-pitchblade to be placed substantially in the wake of a fixed blade, to guidethe flows of gas radially towards the central axis of rotation of theturbine. Each variable-pitch blade is mounted at a distance from theaxis of the cups such that this axis of rotation is positioned facingthe pressure surface of the blade and substantially closer to thetrailing edge than the leading edge of each blade.

Under these conditions, the blades are mounted on the cups at the pointwhere the aerodynamic load is greatest because of the difference inmaximum pressure between the pressure and suction surfaces of the blade.

The incidence of the flows of air is adapted by the blade pitch controlmeans to allow the airflow demanded by the operating point to be matchedwith the flow passing into the turbine in accordance with this demand.Adaptation such as this undoubtedly leads to a loss of efficiency andperformance of the turbine taken in isolation—because of the reductionbrought about by the matching process—but it does optimise thethermodynamic cycle of the turbine engine. In the particular case of theturboshaft engine, the specific fuel consumption is reduced by matchingthe flow rate.

According to specific embodiments, the leading edge of eachvariable-pitch blade has a thickness substantially greater than that ofthe trailing edge and an aerodynamic curved shape optimised forabsorption of a wake of air generated by the blade of the fixed arrayfacing it. In particular, the average thickness of the portion ofvariable-pitch blade between the mounting cups is substantially lessthan the thickness of the remainder of the blade located on the leadingedge side. Furthermore, the blades pivot between two extreme positionsabout a reference position corresponding to 100% of the aerodynamic flowarea: a closed position cutting off the air flow, corresponding to 0% ofthe reference flow area, and an open position of maximum air-flowopening, corresponding to 150% of the reference flow area.

Advantageously, fixed pitch blades are thick enough to ensure thepassage of structural loads. Adequate passage of structural loads makesit possible to limit play and misalignment between the cups and thecasings, and therefore to limit deteriorations in performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent on reading the following description, with reference to theattached drawings, in which, respectively:

FIG. 1 is a diagrammatic view in partial axial section of an example APUfitted with a nozzle according to the invention;

FIG. 2 is a perspective view of the turbine with the nozzle mounted on afirst side plate;

FIGS. 3 a and 3 b are views in partial section of the nozzle accordingto the invention, in a wheel plane and in a longitudinal plane of theturbine along its axis of rotation;

FIG. 4 is a diagram of static pressure exerted on the suction andpressure surfaces as a function of the curvilinear abscissa of a blade,and

FIG. 5 is a view of nozzle blades in a wheel plane in the referencepivoting position and different positions.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

With reference to the general diagrammatic view in FIG. 1, an exampleAPU 1 comprises a gas generator 10 composed of a centrifugal compressor11, a combustion chamber 12 and a turbine 13, the turbine driving thecompressor in rotation via a transmission shaft 20 about the centralaxis X′X. The gases leaving the chamber are expanded in the turbine 13,which also provides power to the equipment. The residual gases thenleave via an exhaust pipe 30.

This power is delivered via a through shaft 20 to an accessory gearbox 3connected to said shaft 20. The accessory gearbox 3 drives, byappropriate speed adaptation means (pinions, reduction gears etc.) thepower plant accessories of the APU and auxiliary equipment 4 specific tothe functioning of the aircraft: alternator, injector, fuel pump, loadcompressor, hydraulic pump etc.

In operation, a throttle governor 5 adjusts the airflow F coming from anair inlet 6, to be compressed in the compressor 11. The compressed airis mixed with the fuel in an injector 15 fitted to the chamber 12. Afterexpansion in the turbine 13, the gases G are ejected into the exhaustpipe 30.

In the example illustrated, the power turbine 13 is a connected turbine.In other examples, the power turbine can be a free turbine or anotherturbine of some attached equipment, linked to the accessory gearbox 3.

The turbine 13 is illustrated in greater detail in the perspective viewin FIG. 2. This inward-flow turbine comprises a mobile impeller 22fitted with vanes 23 and a fixed nozzle 7 mounted on the periphery ofthe impeller 22 on appropriate casings, only the casing 7 a beingillustrated in this FIG. 2 (see the casings 7 a and 7 b in FIG. 3 b).

The radial turbine 13 is fitted with a volute 21—a semi-volute isvisible in the figure—the diameter of which decreases between its inlet21 a and its end 21 b at the vanes 23. This volute allows a tangentialcomponent of the flow of air to be generated, which makes it possible tolimit the deflection of the flow produced by the nozzle in order tosupply the wheel 22.

According to the invention, the nozzle 7 comprises two arrays of blades,a first peripheral array G1 with fixed blades 2 a, for keeping the wallsparallel, and a second array G2 with orientable blades 2 b, foradjusting the flow area. The airflows then drive in rotation the vanes23 and the shaft 20 rigidly connected to the impeller 22.

FIGS. 3 a and 3 b, in the respective sections BB and AA, illustrate theorganisation of the arrays G1 and G2, and their fixed 2 a and orientable2 b blades, in the space separating the two assembly casings 7 a and 7b. The fixed blades 2 a are rigidly connected to the casings 7 a and 7b. Their dimension defines the spacing “e” between these casings, inother words the width of the space E between the parallel casings 7 aand 7 b. The blades 2 a are advantageously thick enough to ensure thepassage of structural loads between the casings 7 a and 7 b.

The ends of each blade 2 b are rigidly connected to the circular,parallel cups 24 a and 24 b, arranged in opposite housings 25 a and 25 bformed in the casings 7 a and 7 b. The blades 2 b are mounted at adistance from the geometric axis of rotation R′R passing through thecups 24 a and 24 b at their centres 2A and 2B. The cups are hereperpendicular to the pressure and suction surfaces of each blade 2 b, Fiand Fe.

Each blade 2 b is capable of being driven in rotation about thegeometric axis R′R by means 40 for controlling the variable pitch of theblades, particularly during the transient phases of the aircraft. Thesecontrol means comprise a stem 41 rigidly connected to the cup 24 b,coupled to mechanical links (arms, pinions, bearings) linked to electricor electromagnetic actuators 42. A single actuator can be configured forall the blades.

The actuator(s) are driven by a central processing unit 50 for enginecontrol. The control can be numerical, electronic or hydromechanical.The incidence of the flows of air defined by the orientation of theblades 2 b is adapted by the control means 40 so as to allow adjustmentof flow rate. In the example illustrated, a pressure sensor 45 providesdata to the central processing unit 50, which regulates the opening andclosing of the blades 2 b of the nozzle 7 via the control means 40.

Each of these blades 2 b has a trailing edge Bf and a leading edge Bafor the flow of air, linked to the faces Fi and Fe of the blade 2 b. Theleading edge Ba of each blade 2 b of the second array is locatedsubstantially in the wake of a fixed blade 2 a of the first array, so asto guide the air flows radially towards the central axis of rotation X′Xof the turbine 22. The wake of a fixed blade corresponds to theaerodynamic trace that it leaves in an undisturbed flow. This wakedefines a highly disrupted low-speed area.

Each blade 2 b is mounted off the axis R′R and is off-centred such thatthe axis of rotation R′R is positioned facing the pressure face Fi ofthe blade 2 b and substantially closer to the trailing edge Bf than theleading edge Ba of each blade 2 b.

Under these conditions, the cups 24 a and 24 b are positioned at thepoint where the aerodynamic load is greatest because of the differencein maximum pressure between the pressure and suction surfaces of theblade. FIG. 4 illustrates the variation in static pressure Ps as afunction of the curvilinear abscissa Ac corresponding to each of thefaces Fi and Fe of a blade 2 b.

A maximum pressure variation is therefore located in the hatched area Z,in the blade portion 2 p situated inside a space “E” delimited by thecups, on the trailing edge side Bf of the blade 2 b. The cups eliminateany play in the area Z where the effect of play is greatest. Theoptimised choice of position of the axis of rotation R′R, offset towardsthe trailing edge Bf, makes it possible to limit the clearance of thetrailing edge Bf itself with respect to its position relative to theleading edge of the vanes 23 of the turbine 22, while limiting themechanical torque required to counteract the aerodynamic torque linkedto the blade and therefore to optimise the absorption of aeromechanicalconstraints.

The leading edge Ba of each blade 2 b has a thickness substantiallygreater than the trailing edge Bf, and an aerodynamic curved shapeoptimised for absorption of a wake of air generated by the blade of thefixed array facing it. In particular, the average thickness of theportion of blade 2 p (in dotted lines in the figure), between theportions of the pressure Fi and suction Fe faces, is substantially lessthan the thickness of the remainder of the blade 2 b located on theleading edge side Ba.

The rotation of the blades 2 b is advantageously limited by an amplitudeof pivoting between two extreme positions. FIG. 5 illustrates theextreme positions 2 b _(sup) and 2 b ₀ about a reference position 2 b_(ref) corresponding to 100% of the aerodynamic flow area. The extremeposition 2 b ₀ corresponds to the complete closing of the flow area. Theposition 2 b _(inf) corresponds to a closed position, with 70% of thereference flow area, intended for low load demands. The position 2 b_(sup) corresponds to the open position, with 150% of the reference flowarea, intended for high load demands.

The invention is not limited to the examples described and illustrated.For example, it is possible to carry out the spacing of the mobileblades solely by mechanical adjustment, whether individual orcentralised, or by electrical or electronic control, with or withoutnumerical control.

1-8. (canceled)
 9. A radial turbine nozzle for a turbine engine,rotating about a central axis, comprising: a first annular array offixed blades and a second annular array with a same number ofvariable-pitch blades, the fixed blades and variable-pitch bladesincluding pressure and suction faces, each variable-pitch blade of thesecond array is rigidly connected to cups extending at each end of thevariable-pitch blade, is configured to be driven in rotation by controlmeans about a geometric axis connecting centers of the cups, andincludes a trailing edge and a leading edge of gas flows linked to thesuction and pressure faces, wherein each variable-pitch blade is mountedat a distance from an axis of rotation of the cups such that the axis ofrotation is positioned facing the suction face of the variable-pitchblade and substantially closer to the trailing edge than the leadingedge of the variable-pitch blade.
 10. A radial turbine nozzle accordingto claim 9, wherein the leading edge of each variable-pitch blade islocated substantially in a wake of a fixed blade so as to guide the gasflows radially towards a central axis of rotation of the turbine.
 11. Aradial turbine nozzle according to claim 9, wherein the leading edge ofeach variable-pitch blade has a thickness substantially greater thanthat of the trailing edge and an aerodynamic curved shape optimized forabsorption of a wake of air generated by a fixed blade of the fixedarray facing it.
 12. A radial turbine nozzle according to claim 9,wherein an average thickness of a portion of a variable-pitch bladebetween the mounting cups is substantially less than a thickness of aremainder of the variable-pitch blade located on a leading edge side.13. A radial turbine nozzle according to claim 9, wherein thevariable-pitch blades are configured to pivot between two extremepositions about a reference position corresponding to 100% of anaerodynamic flow area, a closed position cutting off air flow,corresponding to 0% of the reference flow area, and an open position ofmaximum air-flow opening, corresponding to 150% of the reference flowarea.
 14. A radial turbine nozzle according to claim 9, wherein thefixed-pitch blades are thick enough to ensure passage of structuralloads.
 15. A radial turbine nozzle according to claim 9, wherein theradial turbine is one of a turboshaft engine turbine, an auxiliary powersource of an aircraft, or a turbocharger.
 16. A radial turbine nozzleaccording to claim 9, wherein the radial turbine is fitted with avolute, a diameter of which decreases between its inlet and its end atvanes.